This invention relates to a threaded joint for oil well pipes or gas well pipes which are buried deep in the ground for recovery of natural gas or crude oil from underground. In particular, it relates to a slim-type threaded joint for oil well pipes having a small outer diameter of the joint.
Today, wells for natural gas and crude oil are becoming deep ones having a depth reaching several thousand meters. Threaded joints are widely used for connecting oil well pipes and gas well pipes (collectively referred to below as oil well pipes) for use in such oil wells and gas wells.
In recent years, as the price of natural gas and crude oil has been decreasing, digging of new wells has been performed with a decreased bore diameter of the wells in order to decrease the cost of well development. Reuse of dried-up old wells has also been employed for cost saving using the side track technique in which a hole is formed in the casing pipe of an old well and a tubing pipe is inserted through the hole and advanced to an oil-bearing layer.
In these situations, it is effective to use a slim-type threaded joint, which is a joint having a small outer diameter.
Such a slim-type threaded joint for oil well pipes is disclosed in JP-A 10-89554 (1998). It is an integral-type threaded joint for oil well pipes having threaded portions for threaded engagement of a pin portion and a box portion, metal seal portions for maintaining gastightness, and torque shoulders for regulating the fastening torque. The strength of the torque shoulder portions is made higher than that of other portions of the joint and of the oil well pipe body with the object of increasing resistance to compression. The Japanese publication shows an example in which a torque shoulder portion is located at the center of a threaded portion.
Another form of a slim-type threaded joint is shown in French Patent 7622543. In this case, the outer diameter of the joint is decreased by expanding one end of an oil well pipe body to form a threaded box portion of the joint and swaging the other end of the pipe to form a threaded pin portion thereof.
It is true that with such conventional technology, it is possible to make the outer diameter of a joint slimmer. However, there is a limit to the effect which can be obtained with the above-described conventional slim-type joints, and they have the following problems.
First, the area ratio of the smallest cross-sectional area of the joint which is the critical cross section thereof to the cross-sectional area of the oil well pipe body is small, so the joint strength is low.
In addition, the conventional slim-type joints are not adequate in compression resistance, bending resistance, and over-torque resistance.
The object of the present invention is to provide a threaded joint which can guarantee a joint efficiency which is sufficient for practical use in deep wells, which has a decreased joint outer diameter, and which has an improved bending resistance, compression resistance, and over-torque resistance which are necessary for a side track design.
As a result of various investigations aimed at achieving the object, the present inventors found that by employing a combination of the following structures, a slim-type joint for oil well pipes can be manufactured for the first time which slims down the outer diameter of a threaded joint to at most 1.08 times the outer diameter of an oil well pipe body, which can guarantee a high strength of a joint efficiency of at least 70%, and which has excellent. bending resistance, compression resistance, and over-torque resistance, and they accomplished the present invention.
(i) The thread shape for female and male threads of a joint has a trapezoidal cross section with a load flank angle (xcex1) of 0 to xe2x88x9220 degrees and a stabbing flank angle (xcex8) of 35 to 50 degrees.
(ii) A torque shoulder for regulating the fastening torque is located on the external surface side of an oil well pipe. Preferably it has a shape which is substantially perpendicular to the axis of the pipe.
(iii) A seal portion for guaranteeing gastightness against internal pressure or external pressure or both applied to an oil well pipe is located on the internal surface side of the oil well pipe.
Preferably, a joint is constituted by forming the above-described thread shape on an API EUE (External Upset End) pipe.
Accordingly, the present invention is as follows.
(1) A slim-type integral joint for oil well pipes having female and male threaded portions for threadingly joining a pin portion and a box portion, and metal seal portions located on the internal surface side of oil well pipes for guaranteeing gastightness against internal pressure or external pressure or both applied to the oil well pipes, and further having torque shoulder portions located on the external surface sides of the oil well pipes for regulating fastening torque, characterized in that it has a joint efficiency of at least 70%, the outer diameter of the joint is larger than and at most 1.08 times the outer diameter of the oil well pipe bodies, and the threaded portions have a cross section of a trapezoidal shape with a load flank angle of the threads in the range of 0 to xe2x88x9220 degrees and a stabbing flank angle thereof in the range of 35 to 50 degrees.
(2) A slim-type joint for oil well pipes as described above in (1) wherein the cross-sectional shape of the torque shoulder portions extends substantially perpendicular to the axis of the pipes.
(3) A slim-type joint for oil well pipes obtained by forming threads as described above in (1) or (2) on API EUE (External Upset End) pipes.
(4) A slim-type joint for oil well pipes as described above in any of (1) to (3) wherein the load flank angle (xcex1) of the threads is 0 to xe2x88x9215 degrees.
(5) A slim-type joint for oil well pipes as described above in any of (1) to (4) wherein the stabbing flank angle (xcex8) of the threads is 40 to 50 degrees.
An integral threaded joint for oil well pipes used herein means a threaded joint for oil well pipes which threadingly connects oil well pipes, each pipe having a pin portion at its one end in which a thread is formed on the external surface of the pipe and a box portion at its opposite end in which a thread is formed on the internal surface of the pipe.